Light emitting diode (LED) devices have ever increasing applications. Devices that are capable of generating white light (consisting of R, G, and B components) are particularly interesting because of their potential for replacing conventional light sources, such as light bulbs.
However, some applications have space and illumination considerations that are difficult to overcome even with an LED's relatively small size. For example, some applications, such as flashes for small cameras, e.g., in mobile phones, spot lights, or flash lights, require a large amount of light on target, and yet little room is available for the device. Such applications are particularly problematic because the light distribution from the flash has to fulfill a number of specifications. For example, with a camera flash, it is generally desirable to increase its efficiency so that the illumination intensity in the center of the field of view of the camera to be slightly greater than that towards the edges of the field of view of the camera. Typically, at the horizontal edge of the field of view, the illumination intensity level should be around 60% of the center value, whereas at the corners a value around 30% is acceptable. To increase the illumination intensity level and modify the distribution, secondary optics, i.e., optics between the LED and the scene, are generally used. Without secondary optics, only a fraction, e.g., 20%, of the light emanating from the LED would reach the field of view area, whereas where conventional secondary optics are used approximately 40% of the light reaches the scene. With a conventional flash module, the value of 40% cannot be increased much. If one tries to increase the value, e.g., through collimating the light, other specifications are adversely affected. For example, if one designs secondary optics to increase the fraction of light to values significantly above 40%, the illumination intensity at the edges will decrease causing dark corners in the picture.
Additional problems are encountered with applications that have variable focal ranges. For example, it is desirable for a multi-focus cameras, commonly referred to as zoom lens cameras, to use a variable illumination profile to accommodate the different focal ranges. The use of lighting device with an illumination profile that is optimized for a fixed distance is inefficient in such applications. While the use of mechanical solution are possible, such as the increase or decrease in the source/optic distance, such solutions, however, are difficult to implement and are expensive. Moreover, the inclusion of an electro-mechanical device will require additional space, thereby limiting the space saving advantages of LED devices.